To this same section Gehydrophila have been assigned two remarkable forms of air-breathing “limpet,” _Siphonaria_ and _Gadinia_ (see page 151), and the aberrant _Amphibola_, a unique instance of a true operculated pulmonate. _Siphonaria_ possesses a pulmonary cavity as well as a gill, while _Gadinia_ and _Amphibola_ are exclusively air-breathing. _Siphonaria_ lives on rocks at or above high-water mark, _Gadinia_ between tide marks, _Amphibola_ (Fig. 15) in brackish water at the estuaries of rivers, half buried in the sand. There can be little doubt that all these are marine forms which are gradually becoming accustomed to a terrestrial existence. In _Gadinia_ and _Amphibola_ the process is so far complete that they have exchanged gills for a pulmonary cavity, while in _Siphonaria_ we have an intermediate stage in which both organs exist together. A curious parallel to this is found in the case of _Ampullaria_, which is furnished with two gills and a pulmonary chamber, and breathes indifferently air and water. It is a little remarkable that _Siphonaria_, which lives at a higher tide level than _Gadinia_, should retain the gill, while _Gadinia_ has lost it. The ultimate affinities of the essentially fresh-water groups, _Limnaea_, _Physa_, _Chilina_, cannot be precisely affirmed. The form of shell in _Latia_, _Gundlachia_, and perhaps _Ancylus_, may suggest to some a connexion with the Otinidae, and in _Chilina_, a similar connexion with the Auriculidae. But, in a question of derivation, similarities of shell alone are of little value. It is not a little remarkable, for instance, that we should find a simple patelliform shell in genera so completely distinct from one another in all anatomical essentials as _Ancylus_, _Patella_, _Siphonaria_, _Propilidium_, _Hipponyx_, _Cocculina_, and _Umbrella_.